This is called a “tweelingfiets”. It’s a special Dutch bike for carrying multiple kids and this example seems to date from the 1970’s. At first glance it looks like a modified tandem but it’s not. It’s a “longtail” city bike produced in series around when most of the designers of current longtails and xtracycles were just a glint in their dad’s eye. That doesn’t detract from the Xtracycle and the growing crop of modern longtails, but does show that many ideas come around a few times… especially when it comes to bikes.
I know of at least 4 firms that have produced a tweelingfiets but this type is the oldest. I’m actually not exactly sure who built these but I’m guessing Van Raam who builds or has built frames for a number of firms including: Gazelle, Grimminck, ‘t Mannetje and Utopia Velo.
This particular bike, parked around the corner from my house, has a particularly nice arrangement of child seats. It’s very social for the kids in back. Quite ironically I saw it later today, but with a child facing forward on the rear facing seat.